C64-C128 CP-M Cartridge Interest

I beleive the historay can be traced back to the MCS8i system. This, AFAIK, never had disks, and never ran CP/M. But it's 8080-based and amazingly it has a CPM-like IOBYTE at location 3. This machine has RAM at location 0 by default. The thing is that the 8080 starts executing at location 0, so by puittign RAM there you could enter a program (or at least a jJMP instruction) at location 0, hit reset, and run it. That's what you did on the MCS8i. Since a jump is 3 bytes long, it occupied locations 0.1.2, so location 3 was the first 'free' location, hence used for IOBYTE. To run the ROM monitor on the MSC8i, you toggled C3 00 38 (jump to 0x3800) into the first 3 locations, reset, and let it run. The MDS800, for all it didn't have a front panel, kept the same sort of memory map (for good reasons IMHO). FWIW, I have both machines with manuals... I've seen it done in all sorts of ways. Including a little state machine that picks up the first 3 read cycles after a reset, disables data buffers, etc, so they never go outside the CPU board, and forces a jump instruciton onto the processor data lines. Or, as you say, having a flip-flop that's dset one way on reset to enable a ROM at location 0 and the other way by some other method. One trick was to have the ROM always accessible at the top of memory. On a reset, it was also eneables at locaiton 0 (or it filled the memory map). The frirst instruction in the ROM wasa jump to the 'real' ROM location (let's say it was 0xE000) and the first read cycle with A15 set (or something) cleared the flip-flop, thus making the ROM appear only at the top of the memory map and enabling RAM elsewhere. And there are all sorts of other ways to do it. -tony